As well known from people involved in the art, chromatographic systems rely on the use of valves to allow reproducible sample introduction and various column switching schemes.
Today, in the chromatographic field, there are mainly two types of valves used: the rotary valves and the diaphragm-sealed valves. The rotary type, as the name suggests, uses a rotary movement to switch or divert various flow paths required for a particular application. Description of such valves may be found in U.S. patent application Ser. No. 10/957,560 filed on Oct. 1, 2004 by the same Applicant.
The rotary chromatographic valves are well suited for liquid applications, even if they are also suitable for gas applications. Their design allows the use of various materials to provide inertness or very long lifetime, and relatively high working pressure and temperature which can be required in various liquid chromatography applications. The actuating means used to actuate a rotary valve is generally a pneumatic rotary one or an electrical motor equipped with some gear to increase the torque needed to rotate the valve. In both cases, these assemblies, i.e. actuating means and valve, require a relatively large amount of room in a system. Furthermore, in cases where a pneumatic actuator is used, extra 3-way solenoid valves must be used to allow pneumatic gas to be switched.
In the bulk gas analysis like He, H2, O2, N2, Ar, Kr, Xe, Ne, CO, CO2, CH4, THC, H2O and some other gases, the working pressure and temperature of the chromatographic system is relatively low compared to liquid chromatography. A diaphragm-sealed chromatographic valve could therefore be used since it is generally well suited for gas chromatography. It would so be advisable and beneficial to use diaphragm-sealed valves instead of rotary valves for gas chromatography wherein the design of a rotary valve may probably be overkilled for low pressure and temperature application in gas chromatography.
A diaphragm-sealed chromatographic valve that would take much less room than a rotary system and that could be built at a lower cost, mainly when compared to rotary valves using ceramic material, while providing a long working lifetime would therefore be very desirable.
For the last forty years, many people have designed diaphragm valves for chromatography. Such diaphragm valves have been used in many commercially available gas chromatographs. They are able to be integrated more easily in a gas chromatograph due to their physical size and since the actuator is embedded in the valve itself. These characteristics make them attractive for gas chromatograph manufacturers. However, their performances are poor. For example, the leak rate from port to port is too high and thus limits the system performance. Moreover, the pressure drop on the valve's ports differs from port to port, causing pressure and flow variation in the system. This causes detrimental effect on column performance and detector baseline. Furthermore, many of them have too much inboard contamination. Such valve designs are shown in U.S. Pat. Nos. 3,111,849; 3,140,615; 3,198,018; 3,376,894; 3,387,496; 3,417,605; 3,439,542; 3,492,873; 3,545,491; 3,633,426; 4,112,766; 4,276,907; 4,333,500; 5,601,115 and 6,202,698. The general concept of these valves is shown in FIG. 1.
As illustrated in FIG. 1, the valve 1 is provided with a top block 2 having an interface 4 and a plurality of ports 6. Each of the ports 6 opens at the interface 4 and has an inclined thread passage 8 to connect various analytical fitting and tubing (not shown). At the bottom of the inclined thread passage 8, there is a conduit 10 extending in the top block 2 and opening at the interface 4. The ports 6 are arranged on a circular line on the interface 4 of the top block 2. The interface 4 is advantageously flat and polished to minimize leaks between port and from ambient atmosphere. The valve 1 is also provided with a bottom block 12 and a diaphragm 14, which is generally made of polyimide, Teflon or other polymer material. The diaphragm 14 is positioned between the top block interface 4 and the bottom block 12. The valve 1 is also provided with a plurality of plungers 16, each being respectively arranged to be able to compress the diaphragm 14 against the top block 2 at a position located between two of the ports 6. Preferably, as illustrated, when the valve is at rest, three plungers 16 are up while the three others are down. When the plungers are up, they compress the diaphragm 14 against the top block 2 for closing the conduits made by diaphragm recess 18, so that fluid circulation is blocked. Alternatively, there is fluid flowing between the ports where the corresponding plungers are down. The recess 18 in the diaphragm 14 sits down in the recess 20 made in the bottom block 12, thereby allowing some clearance for fluid circulation. The bottom block 12 keeps the plungers 16 and the actuating mechanism in position.
Referring now to FIG. 2A, there is shown a typical chromatographic application wherein a sample is injected on a separation column to separate the impurities and then to measure them by the integration of successive signal peaks by the detector, as well known in the art. In FIG. 2A, the sample loop SL is swept by the sample gas, while the separation column and the detector are swept by the carrier gas, coming from the valve port #2. To allow this flow path through the valve, the plungers B, D and F are down while the plungers A, C and E are up. The mechanical equivalent of this valve position is shown in FIG. 2B. To do a sample injection, all valve ports must first be isolated from each other to avoid cross port leaks that invariably lead to inaccurate measurements. This is done by setting plungers B, D and F in the up position. The valve analytical flow path and mechanical equivalent of this valve position is shown in FIGS. 3A and 3B. This step is only a temporary intermediate one. Its time duration depends on the actuating mechanism used and the required actuating pneumatic pressure. Then, the sample loop is put in the carrier circuit. This step is generally known as the sampling loop injection position. This is done by moving down plungers A, C and E while keeping plungers B, D and F in the up position. This position is shown on FIG. 4A and the mechanical one in FIG. 4B. In a similar way, to come back in the sampling position which is illustrated in FIG. 2A, the plungers A, C and E are first brought back in the up position. This leads to the intermediate position shown in FIG. 3A, i.e. all plungers up. Finally, the plungers B, D and F are brought back down. So, the valve is now in the position shown in FIG. 2A, i.e. sampling loop filling position. All the patents that we previously referred use this general concept or some slight variation thereof.
Referring again to FIG. 1, the main aspect of this concept is to interrupt the flow between two adjacent ports. For that, the corresponding plunger presses the diaphragm 14, which is then pressed on the interface 4 of the top block 2. Thus, the sealing relies simply on the surface of the plunger defining the area that presses the diaphragm recess 18 on the interface 4. This technique imposes tight tolerances on the surface finish, surface flatness and the plungers' length. Any scratch on the interface 4 or imperfection of the diaphragm 14 will generate leaks. Moreover, the length of all plungers must be the same. Any difference in their lengths will result in leaks, since a shorter plunger will not properly compress the diaphragm against the interface 4. In the prior art, there are some variations of this general concept. The main one relates to the location of the bottom block recess 20. In the past, this recess 20 or its equivalent was located internally in the top block 2, or on its interface 4. U.S. Pat. Nos. 3,111,849; 3,198,018; 3,545,491; 3,633,426 and 4,112,766, which were granted to the same group of people, illustrate this concept. However, as they reported in a more recent valve brochure specification entitled “Applied Automation Company, series 11 diaphragm valve”, this method has been dropped because of a too high cold flow. Cold flow is also often referred to as cross port flow leak. Their latest design, which was commercialized, uses a flat and polished interface 4 on the top block 2 and a recess 20 in the bottom block 12. In this design, the diaphragm 14 has no recess. Moreover, in order to reduce the cold flow, it was also envisaged to use two diaphragms. In fact, as disclosed in U.S. Pat. No. 3,111,849, the use of a “cushion” diaphragm helps to compensate for any slight non-parallelism or length difference of plungers. Other attempts have also been made to correct the non-parallelism, as disclosed in U.S. Pat. Nos. 3,376,894; 3,545,491 and 3,633,426, wherein the use of solid plungers has been replaced with the use of small steel balls.
The concern about plunger length has also been taken into consideration in U.S. Pat. No. 6,202,698, granted to Valco Company, which suggests the use of plungers made of softer material. This allows tolerance reduction for the length of such plungers.
However, such designs still result into too much leak rate between ports since the sealing done by the plungers' pressure is not equal on diaphragm.
Other attempts have been made in the past to eliminate problems caused by plunger tolerance variations. U.S. Pat. No. 3,139,755 discloses a valve wherein no plunger is used. Instead, a hydraulic pressure is used. However, an auxiliary source of pressure must be used since the pneumatic amplification of pneumatic actuating mechanism does not exist. The system, as far as we know, wasn't commercialized. Cross port leaks are still an important problem.
Another design is disclosed in U.S. Pat. No. 3,085,440. In this valve, the diaphragm has been replaced by an O-ring. Nevertheless, cross port leaks are still too high for modern high sensitivity detector.
In brief, in view of the previously mentioned patents, it can be seen that many attempts have been made to try fixing cross port leaks problems and outboard or inboard contamination. All of the proposed designs are quite similar in regard to sealing mechanisms and have the same drawbacks. For example, U.S. Pat. No. 3,140,615, granted in 1964, and U.S. Pat. No. 6,202,698, granted in 2001, do use the same sealing concept in regard to flow switching between ports.
Valco Company did release the DV series valve wherein the diaphragm 14 has an additional recess 18 as illustrated in FIG. 1. The recess 18 sits down in the recess 20 of the bottom block 12. So, when a plunger 16 is in down position, the diaphragm recess 18 sits in the bottom block recess 20, thereby clearing the passage between two adjacent ports, reducing the pressure drop and helping to operate with a low pressure sample.
Finally, it can be seen from the various brochures used to market these valves that the lifetime of these valves is mostly stated in terms of actuations. Most of the time, the number of actuations stated is between 500,000 and 1,000,000. However, it appears that this specification is related to the actuating mechanism and not to the leak rate of the valve. In this aspect, the diaphragm type valve's specifications are not as well defined as the rotary type valve, wherein it is clear that the lifetime of the valve is expressed in terms of leaks.
Besides, a brand new diaphragm valve will often have too many leaks between ports for low level applications. Moreover, it appears that when the valve is at rest for a long period of time, it doesn't perform well when put back in service. This is caused by the diaphragm getting compressed and marked where the plungers press it. It is even worst for valves having fine edge plungers defining a ring type sealing surface.
Thus, the diaphragm type gas chromatography valves of the prior art have several disadvantages: they present too much cross port leaks and too much pressure drop on selected adjacent ports. Moreover, they are difficult to operate when sample pressure is low and they cannot conveniently work with sub-atmospheric sample pressure. Furthermore, they rely on tight tolerance of plungers' length, to minimize cross port leaks.
Therefore, it would be desirable to provide a diaphragm-sealed valve that would overcome the above-mentioned drawbacks of the diaphragm valves of the prior art while being less expensive to manufacture.